1. Field of the Invention
The present invention relates to electron emitting devices, and more particularly, to an electron emitting cathode that operates based upon the triple-junction effect.
2. Description of Related Art
Electron emitting cathodes are used in a variety of devices ranging from cathode ray tubes for display purposes to sophisticated amplifiers used in communication and radar systems for amplifying radio frequency (RF) or microwave electromagnetic signals. For example, it is well known in the art to utilize an electron emitting cathode within a traveling wave tube (TWT), klystron, or other microwave device. In these devices, electrons originating from the electron emitting cathode are focused into a beam and caused to propagate through a tunnel or a drift tube generally containing a RF interaction structure. A RF wave is made to propagate through the interaction structure so that it can interact with the electron beam that gives up energy to the propagating RF wave. Thus, the device may be used as an amplifier for increasing the power of a microwave signal. At the end of its travel, the electron beam is deposited within a collector or electron beam dump, which effectively captures the remaining energy of the spent electron beam. The electron beam may be focused by magnetic or electrostatic fields in the interaction structure of the device to prevent the electron beam from expanding due to space-charge forces and to permit it to effectively travel from the electron gun to the collector without current lost in an undesirable fashion to the interaction structure.
The electron emitting cathode may include some form of heater, such as an internal heater disposed below the cathode surface, that raises the temperature of the cathode surface to a level sufficient for thermionic electron emission to occur. Alternatively, the cathode may be made to produce electrons without the aid of a heater, such as for a cold-cathode gas tube where the electrons are produced by bombardment of the cathode by ions and/or by the action of a localized high electric field. When the voltage potential of an anode spaced from the cathode is made positive with respect to the cathode, electrons are drawn from the cathode surface and caused to move toward the anode.
In addition to the desired electron emission from the cathode in vacuum electron beam devices, there is often undesirable emission from negative electrodes of the devices. In a typical vacuum electron beam device, such as the TWT or klystron noted above, a significant weak point from an electrical breakdown perspective is the interface between a metal, an insulator, and a vacuum. This interface is referred to as a xe2x80x9ctriple junctionxe2x80x9d (i.e., metal-insulator-vacuum) and is illustrated in FIG. 1. The triple junction has been positively identified as a source of field emission electrons in vacuum electron beam devices. The inventor first encountered enhanced field emission from a triple junction in the early 1960""s when attempting to build a very large, high power klystron. FIG. 2 shows a portion of the prior art klystron that uses an insulator 10 that is cylindrical in shape, approximately twelve inches in diameter and eight inches long, and made of alumina ceramic. The insulator 10 has a negative electrode 12 at one end and a positive electrode 14 at the other end and is immersed in a magnetic field 16 with symmetry about an axis 18 of the insulator 10. The insulator 10 is brazed to the negative and positive electrodes 12 and 14, respectively, after metalizing the ends of the ceramic using the molybdenum-manganese process commonly used to make vacuum-tight brazes between a ceramic and a metal. The magnetic field 16 is stronger at the positive electrode 14 than at the negative electrode 12 so that electrons 20 following a trajectory from a triple junction at the negative electrode 12, upon hitting the positive electrode 14, impinge on a circle having a diameter that is smaller than the diameter of the insulator 10. The insulator 10 was intended to hold off 200-300 kilovolts (kV) DC, but at voltages of approximately 150 kV to 200 kV, an electronic discharge was found to occur between the positive and negative electrodes 12, 14. The power was such that melting would occur at the above-referenced smaller circle on the positive electrode 14 and the electronic discharge would develop into a full-fledged arc.
The personnel at the Lawrence Berkeley Laboratory at the University of California were having similar problems, but with lucite cylinders located between essentially flat metal plates that had been glued to the ends of the lucite cylinder with acrylic cement. Using pinholes to collimate the beam of electrons, they had also discovered that the electrons were coming from the junction between the lucite cylinder and the negative electrode, in other words, at the triple junction. A solution that developed in the electron device community and the physics community was to place short metal cylinders with rounded edges inside and outside the insulating cylinder in such a way that the contact between the insulator and the metal is shielded from electric fields. This solution was applied to the klystron of FIG. 2, as shown in FIG. 3, with the placement of short metal cylinders 22, 24 that act to shield the triple junction from the electric fields. This solved the arcing problem.
Over the years, various theories have been proposed as to the cause of the electron emissions near the triple junction. One such theory is stated by H. Craig Miller, in a paper entitled xe2x80x9cSurface Flashover of Insulators,xe2x80x9d presented at the Workshop On Transient Induced Insulator Flashover In Vacuum (Aug. 24-25 , 1988), sponsored by the Microwave and Pulsed Power Thrust Area of the Lawrence Livermore National Laboratory (CONF-8808171). The Miller paper dealt at length with the initiation of flashover near the triple junction at the negative end of insulators. Miller appeared to support the idea that the enhanced emission of electrons from the triple junction was due to a crack between the insulator and the metal, which produced high electric fields at the surface of the metal. For example, when the insulator is mechanically held in place, a crack would exist at the union between the insulator and the metal. Nevertheless, this theory was refuted by others with experience with brazed ceramics, which generally had no cracks at the union.
Another theory is that an electric field concentration caused by the edge of the metalizing is the source of the problem. For example, during the process of metalizing the surface of the insulator and brazing it to the metal, a fillet of braze material on the surface of the insulator unavoidably forms. This theory in turn is contradicted by the experience with lucite insulators that have no fillet of braze material.
In summary, it is known that electron emission does occur at the triple junction, but no hypothesis that fully explains the triple junction effect has been proposed. It would be very advantageous to avoid the undesired consequences of triple junctions and to provide a cathode that utilizes the triple-junction effect to achieve desired electron emission. The triple-junction cathode would be able to provide electron emissions, such as for an electron gun in an electron beam device, display devices or other devices utilizing emitted electrons in their operation.
In accordance with the teachings of the present invention, an electron emitter is provided that is based upon the triple-junction phenomena. The electron emitter is based on the hypothesis that, even with the plain parallel equipotentials and parallel electric field lines that would exist between two plain parallel metal plates separated by a cylindrical dielectric insulator, the electric displacement vector and consequently, the surface charge under the ends of the insulator, will be higher than the surface charge outside of the region contacted by the insulator. In theory, there will be an abrupt step function in the surface charge density in the conduction band of the metal at the edges of the insulator. Nevertheless, there is reason to believe that because of the thermal motion of electrons within the metal, the charge density discontinuity cannot, in fact, be abrupt, but rather will reflect the thermal motion of the electrons. For this reason, just outside the insulator, most of the time, there will be greater electron density in the conduction band of the metal than is required to terminate the electric displacement vector. Therefore, it is likely that some of these electrons will escape from the metal into either vacuum or air and the electron emitter, designed based on this phenomena, may take advantage of the electron emission that occurs at or near the triple junction.
In a first embodiment of the present invention, an electron emitting cathode comprises a cathode body having an emitting surface for emitting electrons. A ferroelectric material is impregnated within the cathode body such that the ferroelectric material enhances the emission of electrons from the emitting surface. The cathode body may comprise a tungsten matrix material and the ferroelectric material may comprise a barium titanate, lithium niobate material and/or other known ferroelectrics.
In a second embodiment of the present invention, a method of making an electron emitting cathode comprises selecting an appropriate base material, forming a cathode body from the selected base material having an emitting surface for emitting electrons, and determining an appropriate insulative material to combine with the base material. The emitting surface produces higher electron emissions as the dielectric constant of the insulative material increases. The base material and the insulative material are then combined. The step of combining may further comprise the step of coating the base material with the insulative material or impregnating the base material with the insulative material.
In a third embodiment of the present invention, an electron emitting cathode comprises a first metallic layer having an emitting surface for emitting an electron beam. A second metallic layer is spaced from the first layer and has a plurality of apertures. A high dielectric constant material is provided between the first and second layers and has a plurality of apertures in substantial alignment with the apertures of the second layer. The first and second layers may comprise a metal material and the first layer may comprise a tungsten matrix material. The high dielectric constant material may comprise a ferroelectric material such as barium titanate, lithium niobate and/or other dielectric material. The high dielectric constant material may comprise an individual layer or may be a coating applied to the first layer. The shape of each aperture may comprise a rectangle, a hexagon, a triangle, a circle, or any other grid-like, random or geometric pattern.
In a fourth embodiment of the present invention, an electron beam device comprises a triple-junction cathode that emits electrons focused into a beam. A collector spaced from the cathode is adapted to collect spent electrons from the beam. A radio frequency interaction section is provided between the cathode and the collector and is adapted to cause an interaction between a radio frequency signal and the electron beam. An anode is provided between the radio frequency interaction section and the cathode and is adapted to draw the electron beam from the cathode. The electron beam device may further comprise at least one of a klystron, a traveling wave tube, a triode, a tetrode, a pentode or other gridded structures.
A more complete understanding of the thermionic electron emitter that is based upon the triple-junction effect will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.